As a magnetic core (core material) for electromagnetic parts (such as motors) to be used with an alternating current, a stack of electrical steel sheets (electromagnetic steel plate) such as electromagnetic soft iron or silicon steel plate were used conventionally. However, they have recently been replaced by a dust core produced by compacting an iron-based soft magnetic powder and then subjecting the resulting green compact to stress relief annealing. Compaction molding of an iron-based powder increases the degree of freedom for designing the shape of a dust core, thereby facilitating production of a core having even a three-dimensional shape. It therefore enables miniaturization or weight reduction of cores compared with those obtained by stacking electromagnetic steel sheets.
Compared with a stacked core obtained by stacking electromagnetic steel sheets, a dust core produced by compacting an iron-based powder has a low iron loss, for example, at a high frequency bandwidth of 1 kHz or more, but is likely to have a more iron loss than that of a stacked core under driving conditions under which a motor is in operation (for example, at a drive frequency of a few 10 Hz to 1 kHz and a flux density of 1 T (Tesla) or more). This iron loss (that is, an energy loss upon magnetic conversion) is known to be expressed by the sum of a hysteresis loss and an eddy current loss, provided that the range is where changes in magnetic flux inside the material are not accompanied by relaxation phenomena (magnetic resonance, etc.) (refer to, for example, SEI TECHNICAL REVIEW NO. 166, published by Sumitomo Electric Industries, March, pp. 1-6(2005) (Non-patent Document 1)).
The hysteresis loss is thought to correspond to the area of a B-H (flux density—magnetic field) curve. Factors having an influence on the shape of this B-H curve and governing the hysteresis loss include, for example, a coercive force of a dust core (loop width of the B-H curve) and the maximum flux density. Since the hysteresis loss is proportionate to a coercive force, it is only necessary to decrease the coercive force in order to decrease the hysteresis loss.
The eddy current loss is, on the other hand, the Joule loss of an induced current accompanying the electromotive force produced due to electromagnetic induction in response to changes in the magnetic field. This eddy current loss is thought to be proportionate to the speed of an electromagnetic field change, that is, the square of the frequency. The smaller the electrical resistance of a dust core or the greater the area where an eddy current flows, the greater the eddy current loss. This eddy current can be roughly classified into an in-particle eddy current flowing inside individual iron-based powder particles and an inter-particle eddy current flowing between ion-based powder particles. If the individual iron-based powder particles are completely insulated therebetween, no inter-particle eddy current is produced and an eddy current consists only of in-particle eddy current, leading to a decrease in an eddy current loss.
In the iron loss, the hysteresis loss is usually dominant to the eddy current loss at a low frequency bandwidth (for example, from a few 10 Hz to 1 kHz) at which a motor is in operation so that a decrease in hysteresis loss is required. Stress relief annealing performed typically after compaction releases strain introduced upon compaction, leading to a decrease in iron loss, particularly, a hysteresis loss. But, stress relief annealing cannot reduce the hysteresis loss without limitation so that a further device for decreasing the hysteresis loss is required.
The Non-patent Document 1 discloses a technology for providing a magnetic powder with low coercive force by enhancing purity and decreasing in-particle strain as a technology for further decreasing the hysteresis loss of a dust core. This Non-Patent Document 1 also discloses improvement in properties, paying attention to effects produced by the improvement of an insulating film for providing a green compact with an increased density, increased electrical resistance, and improved heat resistance. This technology does not however include a consideration on the form of impurities in an iron-based powder. In addition, this technology lacks versatility, because it is necessary to use a high-purity iron-based powder obtained by reducing the impurity content inevitably contained therein and commercially available iron-based powders are not suited for use.
In Japanese Patent Laid-Open No. 2010-10673, disclosed is, as a controlling technology of the form of impurities in the iron-based powder, that is, an inclusion/precipitate, a technology of controlling the composition and dimension of the precipitate, enlarging the precipitate, and thereby improving the magnetic properties. Described specifically, the magnetic properties are improved by precipitating particles composed mainly of oxygen and at least one element selected from the group consisting of Nb, Ta, Ti, Zr, and V and having an average particle size of 0.02 μm or more but not more than 0.5 μm, taking out gas impurities such as O, C, and N from a parent phase of a Fe powder, and thereby cleaning the iron-based powder. This technology has a limit in improving the magnetic properties because it is a technology of producing a precipitate/inclusion that deteriorates magnetic properties.
Japanese Patent Laid-Open No. 139739/1999 proposes a technology of providing a dust core having improved magnetic properties when used under DC magnetization conditions by specifying the chemical component composition of pure iron and an area ratio of non-metallic inclusions. In this technology, the area ratio (dA+dB+dC) of non-metallic inclusions, which is specified in JIS-G0555, is defined as 0.1% or less. This document refers only to the control of an area ratio of inclusions but not to the influence of the dimension of inclusion particles, which is not sufficient for decreasing an iron loss. In addition, use of a dust core only under DC magnetization conditions is assumed in this document so that the above-described improving technology cannot be applied to a dust core used under AC magnetization conditions.
Japanese Patent Laid-Open No. 2007-92162, on the other hand, proposes a technology of providing a dust core having improved magnetic properties by controlling an impurity content in iron powder, the number of crystal grains, hardness, and the like. It is disclosed in this technology that a dust core can have improved magnetic properties by controlling the number of Si-containing inclusions having a size of 50 nm or more to 70% or more of the total number of Si-containing inclusions. In this technology, the dust core having improved properties can be obtained by controlling the size and composition of the inclusions. Existence of inclusions however limits the improvement of magnetic properties. Moreover, when the number of inclusions is great, the above-described technology is presumed to fail to produce an improving effect of magnetic properties.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-505216 discloses a technology of providing a dust core having a low iron loss by specifying an impurity content, an oxygen content, and a specific surface area, as measured by a BET method, of annealed iron powders. This technology proposes an annealing treatment for reducing the oxygen content of the iron powder, but no consideration is given to inclusions. It is therefore presumed that this technology fails to have an improving effect of magnetic properties due to the influence of inclusions.